Alkali‐Labile, Sodium Borohydride‐Reducible Ganglioside Sialic Acid Residues in Brain

Abstract

Abstract: We have shown that ganglioside internal esters, reduced with sodium borohydride and hydrolyzed with mild acid, form nonulosamine and glycosan, whereas ester‐free gangliosides yield only sialic acid when similarly treated. In an effort to demonstrate the occurrence of ganglioside internal esters in brain tissue, brain homogenates and brain ganglioside fractions were treated with NaB3H4. The gangliosides were then hydrolyzed with mild acid and unlabeled carrier nonulosamine and its glycosan were added. The nonulosamine was purified to constant specific radioactivity. Homogenates and ganglioside fractions, initially treated with alkali and then similarly reduced and analyzed, provided control values. Ganglioside fractions directly reduced consistently gave nonulosamine with higher specific radioactivities than controls. A larger quantity of tissue was processed to allow the isolation of chemically measurable amounts of nonulosamine. The amount of nonulosamine formed by reduction of the crude ganglioside fraction was estimated by isotope dilution analysis. The quantity of nonulosamine formed from reduced untreated ganglioside fractions was about sevenfold that formed from alkali‐treated fractions. These data provide evidence for the existence in brain tissue of ganglioside sialic acid residues in which the carboxyl group is bound in a structure that is alkali‐labile and reducible with sodium borohydride.